Transparent conductors are widely used in the flat-panel display industry to form electrodes for electrically switching the light-emitting or light-transmitting properties of a display pixel, for example in liquid crystal or organic light-emitting diode displays. Transparent conductive electrodes are also used in touch screens in conjunction with displays. In such applications, the transparency and conductivity of the transparent electrodes are important attributes. In general, it is desired that transparent conductors have a high transparency (for example, greater than 90% in the visible spectrum) and a low electrical resistivity (for example, less than 10 ohms/square).
Conventional transparent conductors are typically coated on a substrate to form a patterned layer of a transparent, conductive material, such as indium tin oxide or other metal oxide. Such materials are increasingly expensive and relatively costly to deposit and pattern. Moreover, metal oxides have a limited conductivity and transparency, and tend to crack when formed on flexible substrates.
More recently, transparent electrodes including very fine patterns of conductive micro-wires have been proposed. For example, capacitive touch-screens with mesh electrodes including very fine patterns of conductive elements, such as metal wires or conductive traces, are taught in U.S. Patent Application Publication No. 2010/0328248 and U.S. Pat. No. 8,179,381, which are hereby incorporated in their entirety by reference. As disclosed in U.S. Pat. No. 8,179,381, fine conductor patterns are made by one of several processes, including laser-cured masking, inkjet printing, gravure printing, micro-replication, and micro-contact printing. The transparent micro-wire electrodes include micro-wires between 0.5μ and 4μ wide and a transparency of between approximately 86% and 96%.
Conductive micro-wires can be formed in micro-channels embossed in a substrate, for example as taught in CN102063951, which is hereby incorporated by reference in its entirety. As discussed in CN102063951, a pattern of micro-channels can be formed in a substrate using an embossing technique. Embossing methods are generally known in the prior art and typically include coating a curable liquid, such as a polymer, onto a rigid substrate. The polymer is partially cured (through heat or exposure to light or ultraviolet radiation) and then a pattern of micro-channels is embossed (impressed) onto the partially cured polymer layer by a master having a reverse pattern of ridges formed on its surface. The polymer is then completely cured. A conductive ink is subsequently coated over the substrate and into the micro-channels, the excess conductive ink between micro-channels 60 is removed, for example by mechanical buffing, patterned chemical electrolysis, or patterned chemical corrosion. The conductive ink in the micro-channels is cured, for example by heating. In an alternative method described in CN102063951, a photosensitive layer, chemical plating, or sputtering is used to pattern conductors, for example using patterned radiation exposure or physical masks. Unwanted material (photosensitive resist) is removed, followed by electro-deposition of metallic ions in a bath.
In some applications, it is desirable for conductive elements such as metal wires or conductive traces to appear black. U.S. Patent Application Publication No. 2008/0257211 discloses a variety of metallic colored inks and its contents are hereby incorporated by reference.
Optical attributes such as transparency, contrast, or reduced reflectivity are important for display systems. Mechanical concerns such as flexibility and environmental robustness such as scratch and chemical resistance are also important, especially for touch screens designed for interaction with humans. There is a need, therefore, for improved micro-wire structures that meet these needs.